Researchers have found that a roughly 155-mile-wide (250-kilometer-wide) space rock called Juno has a dusty surface that is only about 45% empty space, yet sheds heat like far looser material.
That contradiction turns a giant asteroid’s outer skin into a test of how dust behaves where gravity is weak and air is absent.
Dust defies expectations
The clue came from ten heat images taken across 60% of Juno’s 7.2-hour spin, enough to follow changing warmth.
Across that rotating surface, Jian-Yang Li, Ph.D., at Planetary Science Institute (PSI) showed that Juno’s dust packed moderately but cooled too easily.
The same heat pattern pointed to dust that should have been much fluffier if ordinary grain contact controlled the heat flow.
That mismatch does not prove one hidden process, yet it narrows where scientists should look next.
Heat below dust
At 0.05 inch (1.3 millimeters), the signal carried heat from material just under Juno’s surface.
Regolith, loose rock and dust covering an airless world, controls how quickly sunlight warms and leaves those layers.
Low thermal inertia, resistance to temperature change, means heat does not travel well between nearby grains.
For Juno, Li’s team found the surface held onto heat very poorly, far less than solid rock would.
Telescope catches warmth
High in northern Chile, the Atacama Large Millimeter/submillimeter Array (ALMA), a network of radio antennas, recorded Juno in October 2014.
Those observations resolved Juno at about 37 miles (60 kilometers) across, far finer than earlier asteroid heat surveys.
Because millimeter wavelengths – radio waves far longer than visible light – sense shallow heat, ALMA gave the model more than a surface snapshot.
Even so, the telescope watched only part of one rotation, so local differences remain hard to separate.
Shape rules signal
Juno’s uneven body shaped much of the signal before any dust physics entered the model.
A shape reconstruction, a three-dimensional outline built from telescope data, described Juno as about 155 miles (250 kilometers) wide.
As the asteroid turned, broad faces and narrower ends changed how much warm ground faced Earth.
Shape explained the strongest ups and downs, but it could not erase the strange thermal numbers.
Porosity meets heat
Density offered one side of the conflict because Juno’s index of refraction, how strongly material bends radiation, pointed to 45% open space.
That level of porosity, empty volume inside a material, is loose but not extreme for asteroid dust.
Heat models using ordinary chondrites, common stony meteorites, required far more void space to match the weak heat flow.
The numbers left researchers with a hard physical problem: moderately open dust seemed to behave like extremely open dust.
Grains barely touching
Fine grains offered another clue, since the best-fitting heat values pointed toward particles near 0.0004 inch (0.01 millimeters) across.
Earlier grain-size work showed that asteroid heat behavior depends on particle size, since heat moves through the tiny contact points between grains.
If Juno’s grains touch only lightly, less heat can move downward before the surface cools again.
Repulsive electric charges, forces from uneven electrical buildup, could reduce those contact points, but the model did not prove that mechanism.
Electric depth matters
Another number made Juno look unusual: its dust absorbed millimeter radiation more strongly than lunar-like powder.
The dielectric loss tangent, a measure of electrical absorption, was about 0.4 before correction and near 0.5 after scaling.
That value limits the electric skin depth, the depth radiation can escape from, to roughly 0.004 to 0.06 inch (0.1 to 1.5 millimeters).
“The fitted loss tangent is high compared to the model predictions,” wrote Li and co-authors.
Brightness creates puzzle
Juno also shines oddly around 0.04 inch (one millimeter), where its brightness temperature, a radio estimate of heat, rises instead of fading with longer wavelengths.
Similar behavior appears on Ceres, a dwarf planet in the asteroid belt, but not on most stony asteroids.
PSI’s analysis linked that signal to stronger electrical absorption, which keeps the observed heat closer to the surface.
Composition may play a role, yet Juno’s exception shows dust structure can also change the signal.
Limits guide next
Whole-asteroid measurements kept the result honest because they averaged all visible terrain into one changing signal.
A radiative transfer model – math that tracks escaping radiation – separated shallow heat from deeper warmth.
Even with that tool, the team could not map which exact patches had rougher, cooler, or more absorbent dust.
Future resolved views from ALMA could test whether Juno’s surface varies by longitude or by local slope.
PSI’s model now leaves three linked clues in shallow dust: moderate porosity, weak heat flow, and unusually strong electrical absorption.
Those signals point the way forward – guiding lab tests of meteorite powders and future ALMA observations – but they still stop short of identifying a single underlying cause.
The study is published in The Planetary Science Journal.
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